Separation of linear from branched hydrocarbons using a carbon membrane

ABSTRACT

In a process for separating linear from branched hydrocarbons, the feed is brought in contact with a carbon membrane with critical size from 0.39 to 0.55 nm. The isomer with the larger size remains on the feed side. A membrane can be produced by activating a membrane by reacting it with oxygen and then with hydrogen.

FIELD OF THE INVENTION

The present invention relates to the separation of hydrocarbon isomers.More particularly the invention relates to a method of separating linearfrom branched hydrocarbons using carbon membranes, and to membranestherefor.

BACKGROUND OF THE INVENTION

Carbon membranes, their preparation and their use in the separation ofvarious gases are known in the art, e.g., from U.S. Pat. No. 4,685,940,United Kingdom Patent No. 2,207,666 and European Patent No. 621,071.These membranes have been used for the separation of gas mixturesresulting from various processes. The most common process to which suchmethods have been applied are the separation of nitrogen and oxygen fromair, but the separation of various binary gas mixtures including N₂, He,O₂ and CO₂ have also been carried out. Recently U.S. Pat. No. 5,104,425has taught the separation of hydrogen from hydrocarbons using a carbonmembrane, but those membranes separate based on differences inadsorptive power and surface diffusively of the different molecules suchthat often larger molecules (ethane and propane) permeate better thansmaller molecules (hydrogen). The effect is often weakened or lost athigher temperatures. While the prior art, and specifically theabove-mentioned U.S. Pat. No. 4,686,940, broadly mentions a pore size ofthe carbon membranes in the range of 2.5 Å to 5.0 Å, no membranes withpore sizes of above 3.8 Å have actually been prepared, because theseparation problems contemplated by the prior art did not includedifficult separations, such as the separation of isomers, and did notaddress gases having size which exceeds 3.8 Å. It is clear, therefore,that there is a need for a carbon membrane, characterized by aseparation parameter which is solely dependent on the moleculardimension of the molecules to be separated; namely, the larger is themolecular dimension the lower is its permeability, and that saidmembrane should allow the separation of molecules, dimension of which islarger than 3.8 Å.

The art teaches different methods for separating branched from linearhydrocarbons, which do not involve carbon membranes. U.S. Pat. No.5,069,794 and U.S. Pat. No . 2,924,630 disclose the use of molecularsieves zeolites, which are typically metallo alumino silicates. Thepores in the crystalline structure of these molecular sieves haveappropriate diameters to allow the separation. However, the preparationof said sieves is rather cumbersome, as it involves growing the porouscrystalline structure over an appropriate substrate and then removingthe layer of the crystalline zeolite, or providing the zeolitic barriermaterial the form of a filter cake, trapped between supporting surfaces.Furthermore, the performance of the carbon membrane according to thepresent invention is significantly better. For instance, the selectivityvalue for the separation of 2,2-dimethylbutane from n-hexane accordingto U.S. Pat. No. 5,069,794 is approximately 17. According to the presentinvention, a selectivity value, number of magnitudes of order higher, isobtained when separating 2,2-dimethylpropane from n-pentane.

SUMMARY OF THE INVENTION

It has now been found, and this is an object of the invention, that itis possible to employ carbon membranes not only to separate differentgases, but also to separate isomers of hydrocarbons. This becomespossible because of the size sensitivity of the carbon membrane whichcan differentiate between normal and branched polymers, the former beingof smaller critical dimension independent of the molecular weight andlength. This is unlike other methods, such as fractional distillation,which is sensitive primarily to the molecular weight in a homologousseries such as hydrocarbons. Thus while fractional distillationseparates the petroleum hydrocarbon mixture mainly according to themolecular weight, the carbon membrane separates mainly branched fromnormal hydrocarbons.

This is important in the refining industry where the need for higher andvarious basic chemical precursors and the purification of the productstreams from cat crackers requires separation of such isomers. Theisomers of closet size are those of n-alkanes relative to their singlebranched isomers. (-iso), i.e. which contain a single tertiary carbon.In FIG. 1 it can be seen that there is a difference of 0.5 Å, betweenthese two bids of isomers, independent of their length.

In another aspect, the invention is directed to a method of producing acarbon molecular sieve membrane which is suitable for the separation ofhydrocarbon isomers, and to membranes obtained thereby.

Thus, according to the invention, linear and branched hydrocarbonisomers are separated from one another by using a carbon membrane havingthe appropriate critical size, comprised in the range 3.9 Å to 5.5 Å bybringing a mixture of two isomers to be separated into contact with oneside of the membrane, applying a driving force across the membrane,typically by providing a pressure drop across the membrane, the higherpressure being on the feed mixture side, collecting a permeate richer inthe isomer having the smaller size from the other side of the membrane,and collecting a retentate richer in the isomer having the larger sizefrom the feed side of the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings

FIG. 1 shows the critical minimum molecular dimensions of differentmolecules.

FIGS. 2a-2c illustrate the mechanism of self-activation leading, toinhomogeneous pore size distribution.

FIG. 3 shows an apparatus for tailoring pore size of carbon molecularsieve membrane.

FIG. 4 is a plot of ideal permeance-selectivity parameters of CMSMmembranes prepared for n-C5/i-C5 separations.

FIG. 5 shows the test apparatus for measuring performance of CMSM onpentane isomer mixtures.

FIG. 6 shows the vapor pressures for n-pentane and i-pentane as functionof temperatures.

FIG. 7: illustrates the stability of isomer resolving CMSM membraneswith respect to n-pentane permeance.

DETAILED DESCRIPTION OF THE INVENTION

The method for the separation of linear from branched hydrocarbonisomers according to the invention comprises the steps of:

selecting a carbon membrane having the appropriate critical size,comprised in the range 3.9 Å to 5.5 Å.

providing separating means between the two sides of the membrane, sothat hydrocarbon molecules cannot move from one side of the membrane tothe other, save through the membrane;

providing a mixture of two isomers to be separated and bringing the saidmixture into contact with one side of the membrane;

applying a diving force across the membrane, typically by providing apressure drop across the membrane, the higher pressure being on the feedmixture side;

collecting a permeate richer in the isomer having the smaller size fromthe other side of the membrane; and

collecting a retentate richer in the isomer having the larger size fromthe feed side of the membrane.

As stated, the invention also encompasses a method of producing amembrane suitable for hydrocarbon isomers separation and the membraneproduced thereby. As will be appreciated by the skilled person, suchmembranes can be manufactured by a variety of methods, and the inventionis by no means limited to the use of membranes manufactured by anyspecific route. However, while not wishing to be bound to any particularpreparation method, as stated above, it has been found that it isparticularly advantageous to operate as follows. Firstly, a relativelyclosed membrane (viz., a membrane which has no substantial permeabilityof the selected hydrocarbons) is manufactured, e.g., as described inU.S. Pat. No. 4,685,940. If the membrane is not adequately closed it canbe further closed by the art of chemical vapor deposition (CVD) astaught in U.S. Pat. application Ser. No. 08/213157, now abandoned. Then,the membrane is subjected to activation steps, involving reaction withoxygen followed by hydrogen, as will be described in detail hereinafter.Throughout the manufacture step H₂ /N₂ and O₂ /N₂ selectivities aremeasured The oxygen-hydrogen cycles are continued until the H₂ /N₂, andthen the O₂ /N₂ selectivities drop significantly, at which time thepermeability to hydrocarbons becomes appreciable and the membranecharacteristics can be further adjusted to meet particularpermeability/selectivity requirements for the system involved.

More generally, the three basic thermochemical treatments used invarious orders and combinations for pore development are:

1. Activation treatment with oxygen at temperatures ranging from ambientto 500° C.

2. Treatment in vacuum, inert atmosphere, or reducing atmosphere such asnoble gas, hydrogen, nitrogen or mixtures of them at the range oftemperatures of 300° C.-1500° C.

3. Treatment by chemical vapor deposition (CVD) which coats the membranesurface with a layer. For example, an organic gas vapor that deposits acarbon residue on the membrane surface. In the Table in Example 1,2,2-dimethyl propane was used as CVD material.

Thus, the process according to the above-described preferred embodimentof the invention utilizes O₂, N₂ and H₂ permeabilities as processcontrol parameters in the manufacturing of the membranes of theinvention. As will be appreciated by the skilled person, the majorproblem in the tailoring of membranes of this type is to know when boththe pore mean size and the pore size-distribution function are in thedesirable range. The procedure outlined above, which involves creatingfirst a membrane having good H₂ /N₂ and O₂ /N₂ selectivities and then"ruining" such selectivities by the activation process with oxygen andhydrogen, leads to the formation of membranes which are suitable for theseparation of isomers. This result is surprising since expanding thepores to wider ranges should also expand the nonhomogeneity of the poresize In other words, deeper pore size opening, namely, oxidation withoxygen, that is a controlled burnoff of the carbon material of themembrane, brings about significant weight losses and leads to adisrupted pore structure, and thus to nonhomoreneity. It is well knownfrom elementary organic chemistry that when a certain carbon site in amolecule is attacked through a chemical reaction, the carbon in positionto the attacked carbon will be more reactive, thus it is more likely tobe attacked by further oxidation than a carbon that is remote from thepreviously attacked site. This effect is demonstrated in FIG. 2a-c.

Accordingly, nonhomogeneous pore opening is expected to occur, i.e. whena pore is attacked by oxygen and enlarged, the neighboring carbon atomsbecome vulnerable and thus preferentially attacked further, so that thegeneral tendency is enlarging pores that have been already enlarged inpreference of unattacked pores, thus leading to nonhomogeneous poreopening which leads in turn to low selectivity, as shown in FIG. 2c.

Surprisingly, it was found that the selectivity-permeability combinationof, e.g., normal pentane and 2-methyl butane isomers, is much betterthen the oxygen/nitrogen combination.

The invention is not meant to be limited to the separation of anyspecific isomer pairs. However, without derogating from the generalityof the invention, the isomer pairs listed in Table I below, togetherwith their critical sizes, are of particular interest for the purposesof the invention. According to their critical size, which is thesmallest size that determines the permeability, the hydrocarbon may bedivided into three categories:

1. Normal hydrocarbons where all the carbon atoms are bound to no morethen two other carbon atoms in the molecule. These are the smallest incritical size. Their general formula is: CH₃ --(CH₂)n--CH₃.

2. Isomers that contain at least one ternary carbon. Their generalformula is CH(--R₁)(--R₂)(--R₃) where the Rs are hydrocarbon entitlesthat are by themselves normal or containing ternary but not tertiarycarbons. An example is 2-methyl butane.

3. Isomers that contain at least one tertiary carbon. Their generalformula is C(--R₁)(--R₂)(--R₃)(--R₄) where the Rs are hydrocarbonentities that are by themselves normal or containing ternary or tertiarycarbons. An example is 2,2-dimethyl propane.

                  TABLE I    ______________________________________    Isomer Pair  Critical size range A    ______________________________________    Normal       4.20    Ternary      4.60-4.80    Tertiary     5.45-5.65    ______________________________________

The number of isomers for a cell hydrocarbon increases rapidly with thenumber of carbon atoms. Thus, ethane (CH₃ --CH₃) and propane (CH₃ --CH₂--CH₃) have no isomers, butane has two, pentane has 3 and so forth. Thenumber of theoretical couples becomes tremendous with higherhydrocarbons.

According to one embodiment of the invention, a concentration differenceof one or all the isomers in the feed may be maintained by using a thirdcomponent at the other (permeate) side of the membrane, or by applying apartial or complete vacuum at the other (permeate) side. The above andother characteristics and advantages of the invention will be betterunderstood through the following illustrative end non-limitativedescription of preferred embodiments.

EXAMPLES

In all of the following Examples the permeability characteristics of themembrane with regard to a particular gas are described in terms ofpermeance, which is defined as the flux of that gas STP volume ormoles/(unit area x unit lime)! per unit of driving force, usually thatgas's partial pressure difference across the membrane. In this examplesthe units employed are l(STP) m⁻² hr¹ atm⁻¹.

The membrane pore tailoring is done in an apparatus that can berepresented by the flow sheet in FIG. 3. It provides a feed manifold (1)with appropriate valving and piping to allow controlled rate of flow ofthe gases used for the various termochemical treatments, a vacuum pump(2) for removing gases of the previous thermochemical step beforeintroducing those of the next step, pressure gauges (3) for monitoringfeed and permeate side gas pressures, and an oven (4) with temperatureindicator and controller (TIC-5) for controlling the temperature of themembrane in the presence of the flowing gases, and the membrane moduleitself (6) which is connected to the feed manifold (1), permeatemanifold (7) and the vacuum pump (2) by appropriate valving and piping.In addition, the following elements are shown: CV(calibrated volume),PR(pressure regulator), FC(flow controller).

Example 1 Tailoring membranes to separate between N-C5 and I-C5

Carbon membranes produced by carbonizing a non-melting, such as acellulosic membrane in a controlled atmosphere and a programedtemperature changed scheme was processed according to the thermochemicaltreatment detailed in Table II. In the Table "Neo" stands for2,2-dimethyl propane, "n-C5" stands for n-pentane, "2MeBu" stands for2-methyl butane, "Vac" means that a vacuum was applied, and "H₂ /N₂ "and "O₂ /N₂ " indicate the selectivity of hydrogen and of oxygen,relative to nitrogen respectively.

Permeances of the gases in mixture to be separated (n-C5/2MeBu) arereported in the Table, along with the calculated selectivity and thepermeances of oxygen, nitrogen and hydrogen, as well as their relativeselectivities. From the results it is evident from the permeance data onlines 7,8 that a membrane of good oxygen/nitrogen and hydrogen/nitrogenpermeabilities and selectivities shows negligibly low permeance for allthe hydrocarbons isomers. On the other hand, it is seen from rows 10 and11 that after further activation steps the separation properties (bothhigh permeance and high selectivity) are obtained when several O₂ /H₂treatment steps are applied, and that this membrane has low selectivityfor oxygen/nitrogen or hydrogen/nitrogen separations, as compared to theproperties seen on lines 7,8. This implies that a membrane suitable forisomer separation is of totally different properties than a membranesuitable for separating hydrogen and oxygen from nitrogen.

Comparing the results shown on lines 10 and 11 with those shown in lines13 and 14, it is seen that upon further thermochemical treatment cyclesof O₂ /H₂, the permeance of all gases tested increases at the expense ofselectivity. While the permeance-selectivity values for the pentaneisomers of lines 11 and 14 are of interest for isomer separation, thoseof line 8 are too low in permeance.

It is seen that the production of an isomer separating membrane requiresa certain variety of specific thermochemical treatments. Following thepermeance-selectivity results of is example step by step indicates thatO₂ /H₂ treatments increases permeance and decreases selectivity, whilethe treatment with 2,2-dimethyl propane brings about the opposite.

                                      TABLE II    __________________________________________________________________________    Measurements    permeance (l m.sup.-2 h.sup.-1 atm.sup.-1)                              select.    Step       Tem.  P(atm)                  time        n-C5/         select.                                                select.    No.       (° C.)          gas             in               out                  (min)                      n-C5                         2-MeBu                              2-MeBu                                   O2 N2 H2 H.sub.2 /N.sub.2                                                O.sub.2 /N.sub.2    __________________________________________________________________________    1  280          O2 1 1  75    2  620          H2 1 1  12               1566                                      1300    3  700          Neo             1 Vac                  3                4.3   78    4  280          O2 1 1  20    5  620          H2 1 1  10               66.4                                      12.3                                         935                                            76  5.4    6  280          O2 1 1  10    7  620          H2 1 1  10               425                                      84 2070                                            24.6                                                5.06    8                 14 0.5  28    9  260          O2 1 1  30    10 620          H2 1 1  10               1062                                      504                                         2588                                            5.18                                                2.11    11                149                         3.5  42.57    12 260          O2 1 1  30    13 620          H2 1 1                   1294                                      744                                         2921                                            3.98                                                1.74    14                299                         22.6 13.28    __________________________________________________________________________

Tables III-V reinforce this conclusion, for three additional membranesamples, i.e. that whereas the O₂ /N₂ selectivity has been reduced bysuccessive activations to a range of 1.2-2.2, the n-pentane/2 MeBuselectivity has increased to a range of 13-47

                                      TABLE III    __________________________________________________________________________    Measurements    permeance (l m.sup.-2 h.sup.-1 atm.sup.-1)                              select.    Step       Tem.  P(atm)                  time        n-C5/         select.                                                select.    No.       (° C.)          gas             in               out                  (min)                      n-C5                         2-MeBu                              2-MeBu                                   O2 N2 H2 H.sub.2 /N.sub.2                                                O.sub.2 /N.sub.2    __________________________________________________________________________    1  280          O2 1 1  60    2  620          H2 1 1  15               380                                      128                                         922                                            7.2 2.97    3  700          Neo             1 Vac                  3    4  280          O2 1 1  50    5  620          H2 1 1  15               738                                      415                                         1695                                            4.08                                                1.78    6                 43 4.6  9.848    7  280          O2 1 1  20    8  620          H2                       968                                      691                                         1705                                            2.47                                                1.4    9  200          Neo             1 1  60    10                105                         17   6.176    11 280          O2 1 1  20    12 620          H2 1 1  5                1230                                      976       1.26    13                827                         49   16.88    __________________________________________________________________________

                                      TABLE IV    __________________________________________________________________________    Measurements    permeance (l m.sup.-2 h.sup.-1 atm.sup.-1)                              select.    Step       Tem.  P(atm)                  time        n-C5/         select.                                                select.    No.       (° C.)          gas             in out                  (min)                      n-C5                         2-MeBu                              2-MeBu                                   O2 N2 H2 H.sub.2 /N.sub.2                                                O.sub.2 /N.sub.2    __________________________________________________________________________    1  280          O2 1  1 40    2  620          H2 1  1 10               344                                      48 960                                            20  7.17    3  280          O2 1  1 15    4  620          H2 1  1                  980                                      550                                         1805                                            3.28                                                1.78    5                 50 1.75 28.57    6  200          Neo             Vac                1 60               643                                      295       2.18    7  680          O2 1  1 25    8  620          H2 1  1 10               1167                                      872       1.84    9                 236                         7    33.71    __________________________________________________________________________

                                      TABLE V    __________________________________________________________________________    Measurements    permeance (l m.sup.-2 h.sup.-1 atm.sup.-1)                              select.    Step       Tem.  P(atm)                  time        n-C5/         select.                                                select.    No.       (° C.)          gas             in               out                  (min)                      n-C5                         2-MeBu                              2-MeBu                                   O2 N2 H2 H.sub.2 /N.sub.2                                                O.sub.2 /N.sub.2    __________________________________________________________________________    1  280          O2 1 1  90               3.4                                      0.7       4.86    2  620          HAr             1 1  10               702                                      359       1.96    3  280          O2 1 1  60    4  620          HAr             1 1  10               1489                                      1089      1.37    5  700          Neo             1 Vac                  3                2.8                                      0.6       4.67    6  250          O2 1 1  60    7  620          HAr             1 1  10               9  2         4.5    8  280          O2 1 1  30    9  620          HAr             1 1  10               28 5         5.6    10 800          O2 1 1  45    11 620          HAr             1 1  10               234                                      32 1380                                            43.1                                                7.31    12 280          O2 1 1  60    13 620          HAr             1 1  10               726                                      234                                         2058                                            8.79                                                3.1    14                5.4                         0.7  7.714    15 260          O2 1 1  30    16 620          HAr             1 1  10               1001                                      456       2.2    17                1.6                         0.7  22.86    18 260          O2 1 1  30    19 620          HAr             1 1  10               1162                                      690       1.68    20                85 1.8  47.22    21 260          O2 1 1  45    22 620          HAr             1 1  10               1210                                      835       1.45    23                167                         5    33.4    __________________________________________________________________________

Example 2 n-C5/i-C5 Selectivity achieved without CVD

This example uses a different thermochemical treatment route as shown inTable VI. As can be seen, even though no CVD step is used, but onlyactivation and reduction steps, a practical permeance for n-C5 isachieved (410l m⁻² hr¹ atm ⁻¹) with excellent selectivity (>20).

                                      TABLE VI    __________________________________________________________________________    Measurements    permeance (l m.sup.-2 h.sup.-1 atm.sup.-1)                              select.    Step       Tem.  P(atm)                  time        n-C5/         select.                                                select.    No.       (° C.)          gas             in               out                  (min)                      n-C5                         2-MeBu                              2-MeBu                                   O2 N2 H2 H.sub.2 /N.sub.2                                                O.sub.2 /N.sub.2    __________________________________________________________________________    1  280          O2 1 1  75    2  620          HAr             1 1  10               1088                                      575                                         1843                                            3.21                                                1.89    3  260          O2 1 1  30    4  620          HAr             1 1  10               1735                                      1118                                         2824                                            2.53                                                1.55    5  620            410                         18   22.78    __________________________________________________________________________

Example 3 Results from Actual Mixtures of N-C5/2-MeBu

While Examples 1 and 2 used permeance measurements on pure isomer gasesto obtain calculated ideal selectivities, there was some concern thatcertain effects would interfere with achieving his selectivity inseparations of actual mixtures. The separation of hydrocarbon mixes iscomplicated by the fact that they are much easier to liquefy than nobleor air-like gases. As a result, if feed temperature and pressureconditions are not sufficiently far from those needed for liquefaction,partial condensation of components of the gas mixture can occur withinthe pores of the membrane, even though the mixture remains a gas in thefeed. This can even result in the less permeable component condensingand preventing passage of the more permeable component. This Exampledemonstrates practicality of operating the membrane with mixtures toattain selectivities approaching those calculated from pure gaspermeances.

The experimental setup was as follows:

The module was tested in the apparatus shown in FIG. 5 to measurepermeance and selectivity in the presence of mixtures of pentaneisomers. The same apparatus could be used for measuring the permeance ofthe pure isomers and for measuring mixtures of other alkanes (eg C3-C10)as well.

In order not to consume large quantities of the pentane mixture duringthe course of the experiment, the liquid mixture was kept at 0° C. tomaintain a relatively low vapor pressure. The graph of pentane vaporpressure as a function of temperature is provided in FIG. 6 as areference. The C5 isomer mixture vapor was transported to the membranemodule by a nitrogen sweep gas. In these experiments the feed was on thebore side of the module. A pressure gauge (Pf) was used to measure thetotal pressure of the feed gas and a bubble flow meter (BFM) was used tomeasure the flow rate of the sweep gas in the retentate after removal ofthe condensible hydrocarbons. A similar arrangement was made to measurethe nitrogen flow rate in the feed stream. From a knowledge of the totalpressure of the feed gas stream and the vapor pressure of the pentanesthe partial pressures of all components could be calculated.

A vacuum was pulled on the permeate side to provide driving force. Theisomer mixture in feed, retentate and permeate streams could each becondensed in a cold trap (CT) for subsequent analysis in a gaschomatograph to establish the composition of each stream. The flow rateof the isomer mixture in each stream was determined by measuring theaccumulated weight change in the cold trap for a determined period oftime. The condensed pentane isomers were then mixed with a high boilingdiluent (dodecane) to prevent evaporation and compositional changesbefore delivering the samples for gas chromatography.

In actual practice a stream composed of hydrocarbons without the sweepgas hydrocarbon stream the module temperature must be maintained at atemperature at which the equilibrium vapor pressure of the pentaneisomers exceeds the operating pressure in order to prevent condensationin the pores and loss of penneability. In this work the module wasmaintained at elevated temperatures (80-140° C.) by a cylindrical ovenaround the module housing. The elevated temperature preventedcondensation of the hydrocarbon in the membrane pores and promoted therate of transmembrane transport.

The raw data fox a typical experiment are provided in Table VII. Thetotal amount of alkane in each stream was determined gravimetrically.The relative amounts of each isomer were determined on a GC. Byconverting all raw data into molar flows they were reduced to the massbalance summary found in Table VIII. The molar flow of nitrogen in thepermeate was not measured but calculated by difference between feed andretentate assuming 100% mass balance for nitrogen. The driving force forpentane permeation was calculated from its partial pressure in the feedstream allowing for the axial pressure drop. The permeate pressure wasnegligible under vacuum. From the permeation rate for each of theisomers and the partial pressure driving force, the permeance andselectivity was calculated for each isomer.

                                      TABLE VII    __________________________________________________________________________    Temperature of Pentane Reservoir: 0° C.    Reservoir Isomer Ratio (n-C5:i-C5): 70:30    Diluent in permeate/Retentate Trap: dodecene    Membrane T 100° C.    P°(0°) n-C5 = 185 torr                       # of fibers 10    P°(0°) 2-MeBu = 259 torr                       Length, cm 9                CARRIER                        part.                                                   part.        alkene            sample                Gas Flow                      total                         added                 press.                                                   press.    exp.#        net time                ccNTP/                      press                         diluent                             GC mg                                 GC mg                                     true mg                                          true mg                                               (torr)                                                   (torr)    5a  mgr min min   (torr)                         cc  n-C5                                 2MeBu                                     n-C5 2MeBu                                               n-C5                                                   2MeBu    __________________________________________________________________________    FEED        100.8            3.9 36.7  817                         5   67.5                                 41  62.7 38.1 98.1                                                   59.6    PERM        47.4            64  0     0  5   39  1.5 45.6 1.8  0.0 0.0    REJE        90.9            3.57                34    720                         5   54  34  55.8 35.1 89.6                                                   56.4    __________________________________________________________________________

                                      TABLE VIII    __________________________________________________________________________    Summary of separation of pentane isomer mixture.    EXPT 5a: Mixed Pentanes in N2 Carrier St    STREAM:   Permeate Retentate                              MASS   Permeance    Component          Feed              ←Jv(Mol m.sup.-2 H.sup.-1)→                         % CUT                              BALANCE                                     (Sl m.sup.-2 H.sup.-1 Ar.sup.-1)    __________________________________________________________________________    n-pentane          28.69              1.27  27.88                         4.4% 101.6% 230.9    iso-pentane          17.43              0.05  17.55                         0.2% 101.0% 14.4    N2 Carrier          192.84              14.19 178.65                         7.4% 100.0% 391.7    TOTAL 225.44              15.51 224.09                         6.9% 106.8%    SELECTIVITY (n/iso): 16.1    __________________________________________________________________________

Example 4

This Example shows the membrane permeance for n-pentane is stable overpractical lengths of operating time. A stream of pentane in nitrogencarrier gas was fed through the module as described in Example 3. Thetemperature was maintained at 80° C. Periodically the permeance wasmeasured. The results of this test are given in FIG. 7.

EXAMPLE 5

This Example shows how the selectivity is greatly increased as the sizedifference between the two isomers increases. Neopentane differs fromn-pentane by about 1.2 Angstroms in its smallest molecular dimension.This compares with a difference of only about 0.4 Angstroms betweenisopentane (i.e. 2-methyl butane) and n-pentane.

A hollow fiber made from pyrolyzing a non-melting polymer, with a wallof 10 μm thickness was processed according to the thermochemicaltreatment detailed in Table VII. Permeance of the gases to be separated(n-C5/Neo) are reported in the Table, along with the calculatedselectivity and the fluxes of oxygen, nitrogen and hydrogen, as well astheir relative selectivities. From the results in the Table it is seenthat excellent n-C5/Neo selectivity is obtained at the end of thetreatment.

                                      TABLE IX    __________________________________________________________________________    Measurements    permeance (l m.sup.-2 h.sup.-1 atm.sup.-1)                              select.    Step       Tem.  P(atm)                  time        n-C5/         select.                                                select.    No.       (° C.)          gas             in               out                  (min)                      n-C5                         2-MeBu                              2-MeBu                                   O2 N2 H2 H.sub.2 /N.sub.2                                                O.sub.2 /N.sub.2    __________________________________________________________________________    1  280          O2 1 1  60               914                                      408       2.24    2  620          HAr             1 1  10    3  260          O2 1 1  60               1670                                      1178      1.42    4  620          HAr             1 1  10    5  280          O2 1 1  20               2028                                      1610      1.26    6  800          HAr             1 1  80    7  280          O2 1 1  15               2296                                      1908      1.2    8  620          HAr             1 1  10    9  280          O2 1 1  15               2525                                      2505      1.01    10 620          HAr             1 1  10               2147                                      2087      1.03    11                1280                         0.6  2183    __________________________________________________________________________

Example 6

Example shows that the membrane power to separate isomers is not limitedto C5 isomers but is general for straight vs. branched chainhydrocarbons, as described in the invention. In this Example themembrane selectivity between n-butane and i-butane is demonstrated.

The membrane was prepared in a similar manner to that in Example 1 andthe pure gas permeances were measured after each activation at 80° C.The results are given in Table X.

                  TABLE X    ______________________________________    Ideal Permeances for C4 isomers for CMSM cell #1578/3    Preparation               Permeances (L(STP)/M2-Hr/Atm)    step       n-C4        iso C4   ideal S    ______________________________________    1st activation               82          1.4      59    2nd activation               320         9.1      35    3rd activation               510         26       19.6    ______________________________________

Example 7

In the above Example it was found that the measured values of thepermeance of n-butane after i-butane were lower than if n-butane alonewere measured. This reinforced the concern about adsorption/condensationof the hydrocarbons if the temperatures were not kept above thecondensation point. Therefore in this example, mixtures of butaneisomers were measured to show the practical effectivness of theinvention in separating actual mixtures.

The modules were prepared as described in the previous examples. Thetesting apparatus used was similar to that shown in FIG. 5 with thefollowing modifications:

The N₂ carrier gas and liquid reservoir arrangement were eliminated.Instead CP i-butane and n-butane were fed separately from lecturebottles and mixed in the manifold where they were then fed into themodule. Permeate was accumulated in a cold trap (CT). Its quantity wasdetermined by evaporating and expanding the condensed gas into apreviously evacuated calibrated volume at room temperature. From thepressure the quantity of gas mixture was determined, using the equationof state for gases (n=PV)RT). Removable sample traps of permeate andretentate streams were submitted for GC analysis as were samples of thefeed stream. The results of runs on two different samples are shown inTables XI and XII. They demonstrate the striking performance of the CMSMmembrane in effecting the separation.

All the above description and examples have been provided for thepurpose of illustration, and are not intended to limit the invention inany way.

                                      TABLE XI    __________________________________________________________________________    Temp # of fibers                   2    100° C.         length, cm                   25.8                    Gas              partial                                         partial        trap           trap sample                    Flow total                             by GC                                 by GC                                     press.                                         press.    exp.        Vol           pressure                time                    (ccSTP/                         press.                             X   1-X (torr)                                         (torr)                                             SLm.sup.-2 h.sup.-1 atm.sup.-1                                                      SLm.sup.-2 h.sup.-1                                                      atm.sup.-1    #2  (cc)           (Torr)                (min)                    min) (torr)                             nC4 iC4 n-C4                                         i-C4                                             n-C4     i-C4     select.    __________________________________________________________________________    Feed:  870      6.97 870 60.1%                                 59.7%                                     523 345    Perm:        37.7           365  31  0.56 0   96.9%                                 2.6%                                     0   0   219.7    9.1      24.2    reject        10 720  30  6.47 720 61.2%                                 38.6%                                     441 278    __________________________________________________________________________

                                      TABLE XII    __________________________________________________________________________    Temp # of fibers                   2    100° C.         length, cm                   29                    Gas              partial                                         partial        trap           trap sample                    Flow total                             by GC                                 by GC                                     press.                                         press.    exp.        Vol           pressure                time                    (ccSTP/                         press.                             X   1-X (torr)                                         (torr)                                             SLm.sup.-2 h.sup.-1 atm.sup.-1                                                      SLm.sup.-2 h.sup.-1                                                      atm.sup.-1    #3  (cc)           (Torr)                (min)                    min) (torr)                             nC4 iC4 n-C4                                         i-C4                                             n-C4     i-C4     select.    __________________________________________________________________________    Feed.  863.326  8.03 870 26.0%                                 74.0%                                     226 644    Perm:        37.7           510  30.5                    0.76 0   85.3%                                 14.7%                                     0   0   683.4    36.5     19.3    reject        10 720  30  7.27 720 19.7%                                 80.3%                                     142 578    __________________________________________________________________________

We claim:
 1. A method for the separation of linear from branchedhydrocarbon isomers, comprising the steps of:selecting a carbon membranehaving a pore size, comprised in the range 3.9 Å to 5.5 Å; providing aseparating means between the two sides of the membrane, so thathydrocarbon molecules cannot move from one side of the membrane to theother, save through the membrane; providing a mixture of two isomers tobe separated and bringing the said mixture into contact with a mixturefeed side of the membrane; applying a driving force across the membrane;collecting a permeate richer in the isomer having the smaller size froma permeate side of the membrane opposite to said mixture feed side; andcollecting a retentate richer in the isomer having the larger size fromsaid mixture feed side of the membrane.
 2. A process according to claim1, wherein the feed mixture of isomers is kept in the gaseous state. 3.A process according to claim 1, wherein a concentration difference ofone or all the isomers in the feed is maintained by applying a partialor complete vacuum at the permeate side of the membrane.
 4. A processaccording to claim 1, wherein the isomers to be separated are selectedfrom normal hydrocarbons and hydrocarbons containing secondary, tertiaryor ternary carbon atoms.
 5. A method for the separation of linear frombranched hydrocarbon isomers, comprising the steps of:selecting a carbonmembrane having a pore size, comprised in the range 3.9 Å to 5.5 Å;providing separating means between the two sides of the membrane, sothat hydrocarbon molecules cannot move from one side of the membrane tothe other, save through the membrane; providing a mixture of two isomersto be separated and bringing the said mixture into contact with amixture feed side of the membrane; providing a pressure drop across themembrane, the higher pressure being on the mixture side thereof;collecting a permeate richer in the isomer having the smaller size fromthe permeate side of the membrane opposite to said mixture feed side;and collecting a retentate richer in the isomer having the larger sizefrom said mixture feed side of the membrane.